This invention relates to a snubber circuit for a gate turnoff thyristor, and more particularly to improvements in a snubber circuit for suppressing the abnormal rise of the anode - cathode voltage of a gate turnoff thyristor.
The gate turnoff thyristor (hereinbelow, termed `GTO`) is an element of the so-called self-extinguishing type in which a main current can be interrupted by causing a negative current to flow through a gate. Since it does not require a commutation circuit, it has the advantage that devices can be made smaller in size. It has therefore come into extensive use in chopper circuits, various inverter devices, etc. However, unless the rise rate of the anode voltage of the GTO during the interrupting operation is suppressed below a value prescribed for the GTO element in interrupting the main current (anode current) of the GTO by means of the gate, the turnoff of the GTO will fail, and the GTO will be destroyed on that occasion.
FIG. 1 is a circuit diagram which shows a prior-art example of a snubber circuit for suprressing the anode voltage rise rate (dV/dt) of a GTO. Referring to the figure, the snubber circuit 2 is connected across the anode 7 and cathode 8 of the GTO 1. This snubber circuit 2 includes a diode 3, a capacitor 4 and a resistor 5. The capacitor 4 and the diode 3 are connected in series, and are connected across the anode 7 and the cathode 8. The diode 3 has its anode connected to the anode 7 of the GTO 1 through the capacitor 4 and has its cathode connected to the cathode 8 of the GTO 1 so as to have the same polarity as that of the GTO 1. The resistor 5 is connected in parallel with the diode 3. On the other hand, a gate drive circuit 6 for supplying the gate electrode 9 of the GTO 1 with a gate pulse current for turning it `on` and `off` is connected across the gate electrode 9 and the cathode 8. An inductance 10 connected to the anode 7 is the leakage inductance of a main circuit (not shown).
FIG. 2 is a waveform diagram for explaining the turnoff operation of the GTO 1 shown in FIG. 1. Referring now to FIG. 2, the operation of the circuit in FIG. 1 will be described. When a main current I.sub.A is flowing from the anode 7 toward the cathode 8 of the GTO 1, a gate reverse current I.sub.GR is started flowing from the cathode 8 toward the gate electrode 9 of the GTO 1. Then, the main current I.sub.A begins to suddenly decrease after a certain fixed delay time t.sub.s. At this time, an abrupt spike voltage is generated by the leakage inductance (stray inductance) 10 which exists in the main circuit. As spike voltage is about to be applied across the anode 7 and cathode 8 of the GTO 1, a bypass current temporarily flows through the capacitor 4 via the diode 3, so that a sudden change in the main current is restrained, and the anode voltage rise rate (dV/dt) during the interrupting operation can be suprressed to the prescribed value. The rate dV/dt in this case is roughly expressed by dV/dt=I.sub.GQ /C where I.sub.GQ denotes a turnoff current (the main current immediately before the turnoff) and C denotes the capacitance of the capacitor 4. Charges stored in the capacitor 4 during the interruption of the anode current are quickly discharged through the resistor 5 within the turn-on period of the GTO 1, so that it may be ready for the next turnoff cycle.
Meanwhile, letting f denote the on/off recurrence frequency of the GTO 1 and V.sub.c denote a circuit voltage (the charged voltage of the capacitor 4), a power loss W.sub.s by this snubber circuit 2 is expressed by W.sub.s =(1/2) C.multidot.V.sub.c.sup.2 .multidot.f and is proportional to the capacitance C of the capacitor 4.
The prior-art snubber circuit for the GTO is constructed as described above. Therefore, in a case where the main current I.sub.A and hence the turnoff current I.sub.GQ changes, the capacitance C of the capacitor 4 needs to be set in conformity with the maximum value thereof. Accordingly, there has been the problem that, for a light load whose turnoff current I.sub.GQ is smaller than the maximum value, an excess snubber loss arises on account of the unnecessarily large capacity of the capacitor (the snubber loss is the power loss which occurs due to the charging and discharging of the capacitor and which is mostly consumed by the resistor 5 of the snubber circuit). This has been a serious problem especially in an apparatus of which a short-time overload bearing capacity is required.